Such a tool can typically be provided on a lower end of a drill string that is deployed in a subterranean bore hole, whereby during operation the fluid is pumped in the form of a drilling fluid from surface through a longitudinal channel in the drill string to the tool and essentially back to surface in a return stream through an annular space between the drill string and the bore hole wall. In order to avoid continuous circulation of the abrasive particles through the drill string and the annular space, U.S. Pat. No. 6,510,907 proposes to provide the tool with a recirculation system for separating the abrasive particles from the return stream and re-inserting these particles into the jetting system.
Another such tool is described in International publication WO 02/34653. The recirculation system is based on a helical separator magnet that is concentrically arranged within a support member. The support member is formed by a cylindrical sleeve, of which sleeve the outer surface forms a support surface on which magnetic particles are retained by the magnetic field generated by the helical separator magnet. The separator magnet has a central longitudinal axis about which the separator magnet is rotatable relative to the sleeve.
When the separator magnet is driven into axial rotation, the magnetic particles experience a moving gradient of magnetic field strength perpendicular to the helical groove, which the particles will follow. In this way the particles are transported over the support surface back to the jetting system for re-insertion.
Both prior art tools rely on an efficient transfer of the abrasive particles from the separator magnet into the jetting system. This is of particular importance when a large number of abrasive particles must be recirculated per time unit, because in that case clusters of abrasive particles can cause an obstruction at the abrasive particle inlet of the jetting system. Clustering of the abrasive particles is enhanced by their magnetic interactions.
Moreover the return stream from which the abrasive particles are to be separated normally also contains excavation debris that may include rock grains bigger than an access window of the abrasive particle inlet into the jetting system. Such grains can block the abrasive particle inlet of the jetting system, and thereby hamper the recirculation process of abrasive particles.